diff --git a/clang/lib/Serialization/GlobalModuleIndex.cpp b/clang/lib/Serialization/GlobalModuleIndex.cpp
--- a/clang/lib/Serialization/GlobalModuleIndex.cpp
+++ b/clang/lib/Serialization/GlobalModuleIndex.cpp
@@ -321,7 +321,7 @@
     = *static_cast<IdentifierIndexTable *>(IdentifierIndex);
   IdentifierIndexTable::iterator Known = Table.find(Name);
   if (Known == Table.end()) {
-    return true;
+    return false;
   }
 
   SmallVector<unsigned, 2> ModuleIDs = *Known;